This invention relates to compositions suitable for injection molding of metal powders which contain as a binder paraffin and a fatty acid. The binder of the present invention is easily obtained, is relatively uncomplicated as compared with prior known binders, and is as a result, therefore, safer, more economical, and more easily removed during the dewaxing and sintering operation of green articles made from the metals than prior known binder systems. Use of the fatty acid results in essentially complete wetting of the material during compounding. This results in the resulting green article being uniform in composition throughout.
Sinterable materials may be formed into shapes by various processes. Injection molding is a process wherein a material can be formed into a shape by forcing the material into a mold or die by applying a pressure to the fluidized material. The injection molding process facilitates a rapid and repeated forming of a plurality of articles having a consistent shape with close dimensional tolerances. The injection molding process minimizes the amount of shaping or machining that may be required to produce a finished article.
Many problems are encountered in the injection molding process for particulate materials which require high solids to binder ratio. Some of the problems which make it difficult to make complex shapes of high solids to binder ratio compositions required in many sinterable materials such as metallic materials, are low green strength of the articles as molded, segregation of the binder from the particulate material during molding and poor green body flexibility.
In addition to the above problems, many binder systems are comprised of various types of polymers, and exotic blends of materials that are removed by heat, solvents, and capillary action. The more complicated the binder system, the more complicated, and often more hazardous is the removal of the binder.
U.S. Pat. No. 4,456,713 relates to a composition for injection molding which comprises a finely divided sinterable material and an organic binder comprising a hydrocarbon wax having a melting point of about 40.degree. C. to about 75.degree. C. and a volatilization temperature of about 150.degree. C. to about 450.degree. C. about 5 to 15% by weight of a thermosetting resin having a thermosetting temperature of about 120.degree. C. to about 160.degree. C., and a surfactant. This binder composition is relatively complicated. Generally this makes the binder harder to remove than a simpler binder would be because more components have to be removed and different temperatures are involved. There are problems associated with thermosetting resins such as the possibility of the resin setting up during compounding or curing at room temperature while standing. This leads to problems with reusing sprues and runners and in reprocessing of scrap material.